The first deep-snouted tyrannosaur from Upper Cretaceous Ganzhou City of southeastern China

Tyrannosaurids were the most derived group of Tyrannosauroidea and are characterized by having two body plans: gracile, long-snouted and robust, deep-snouted skulls. Both groups lived sympatrically in central Asia. Here, we report a new deep-snouted tyrannosaurid, Asiatyrannus xui gen. et sp. nov., from the Upper Cretaceous of Ganzhou City, southeastern China, which has produced the large-bodied and long-snouted Qianzhousaurus. Based on histological analysis, the holotype of Asiatyrannus xui is not a somatically mature adult, but it already passed through the most rapid growth stages. Asiatyrannus is a small to medium-sized tyrannosaurine, with a skull length of 47.5 cm and an estimated total body length of 3.5–4 m; or around half the size of Qianzhousaurus and other large-bodied tyrannosaurines in similar growth stages. Asiatyrannus and Qianzhousaurus are sympatric tyrannosaurid genera in the Maastrichtian of southeastern China. Asiatyrannus differs from Qianzhousaurus in that it has a proportionally deeper snout, longer premaxilla, deeper maxilla, and deeper dentary, and the cornual process of the lacrimal is inflated without developing a discrete horn. The different skull proportions and body sizes suggest that Asiatyrannus and Qianzhousaurus likely had different feeding strategies and occupied different ecological niches.


Etymology
The generic name is derived from Asia, and the suffix 'tyrannus' is derived from the Latin word for 'king' or 'tyrant' , to emphasize that this is the new tyrannosaur collected in the continent of Asia.The specific name honors Dr. Xing Xu (Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences), a distinguished dinosaurologist who contributed greatly to the study of dinosaurs from China, including the research of several tyrannosaurs: Guanlong, Dilong, and Yutyrannus [20][21][22] .Dr. Xing Xu has also been a great supporter of the paleontological research and science popularization work of the Zhejiang Museum of Natural History.Holotype ZMNH M30360, housed at Zhejiang Museum of Natural History, Hangzhou, Zhejiang, China.A nearly complete skull and partial disarticulated postcranial skeleton, including caudal vertebrae; right femur, tibia, and fibula; metatarsals, pedal phalanges, and a partial midshaft of the left tibia, fibula, and metatarsals (Figs. 1, 2, 3, 4, 5, 6 and 7, Table 1).

Diagnosis
Asiatyrannus xui is a small to medium-sized tyrannosaurine theropod that differs from other tyrannosauroids in possessing the following autapomorphies: two small, deep fossae located on the lateral surface of the premaxilla just lateral to the anteroventral border of the external naris, a large and sub-rectangular shaped maxillary fenestra, the posterior protuberances of the nasals connect to form two separated medium low ridges, a low ridge-like jugal accessory horn, the ventral margin of the anterior ramus of the jugal curving ventrally anterior to the accessory horn, the lateral surface of the descending process of the postorbital developed the anterodorsally trending fine lineations, the slender, straight, and banded-shaped postorbital bar, with almost straight and parallel anterior and posterior margins in lateral view, and the lateral surangular shelf extends to the posterior end of the surangular.

Ontogenetic age and body size
The periosteal cortex of the right fibula is zonal and exhibits a minimum of 13 lines of arrested growth (LAG) (Fig. 3).The fibular section lacks a medullary cavity similar to the juvenile Tarbosaurus MPC-D 107/7 24 , however, the secondary remodeling is more developed in ZMNH M30360 than MPC-D 107/7 24 .The periosteal bone is incomplete; therefore, the total number of LAGs and the presence of an external fundamental system cannot be determined.Thus, the Asiatyrannus had a terminal age of at least 13 years of life.The outer zones became narrower, indicating a slowdown of growth was occurring, but the animal was still experiencing active growth.The medial half of the fibula had undergone a large degree of secondary remodeling, which obliterated the LAGs (Fig. 3).The narrower outer zone and the highly developed secondary osteons indicate that the holotype of the Asiatyrannus xui is not a full-growth adult individual, but it already passed through the most rapid growth stage.The skull of Asiatyrannus shows several matured cranial morphology: the ventral edge of the maxilla has a distinct convex curvature; the nasals are fully fused with pronounced vaulting and well development of protuberances; the cornual process of lacrimal is strongly inflated similar to the adult Tyrannosaurus and Tarbosaurus 2 ; the dentary is relatively deep.Thus, we interpret the specimen to be a sub-adult that was nearing somatic maturity.The early branching, small-bodied tyrannosauroid Guanlong and Moros reached or neared their adult size at 6-7 years of age 20,25 .By contrast, the large-bodied, Campano-Maastrichtian tyrannosaurines were undergoing exponential stage growth around 14 years old 26 .By comparison, the Asiatyrannus holotype is not a full-growth individual and experienced an earlier exponential growth rate when compared with other large-bodied tyrannosaurines from North America 26 .The holotype's femoral length is around half the length of individuals with a similar number of LAGs 26 , thus indicating Asiatyrannus represents a relatively small-bodied tyrannosaurine compared with other large-bodied Campano-Maastrichtian tyrannosaurines.The length of Asiatyrannus skull and femur is 47.5 cm and 44 cm, respectively.The total body length of Asiatyrannus is estimated from 3.5 to 4 m (Fig. 1).The skull is about 8% longer than the femur, while the same ratio in Qianzhousaurus is 29% since the length of the skull and femur is 90 cm and 70 cm, respectively 10 .

Comparative description
The holotype of the Asiatyrannus xui preserves a nearly complete skull and partial postcranial skeleton.The right side of the skull is well-exposed, but the posterior half of the left side is overlain by the postcranial skeleton.The external texture of most bones is not well-preserved, with the periosteal surface obscured by cracks, breaks, holes, and fractures, resulting from taphonomic processes.

Skull
The posterior half of the left skull is obscured by the caudal vertebrae, and the left mandible is concealed by the hindlimb (Figs. 2, 4), the right side of the skull is well-exposed (Fig. 5).Overall, the skull is well-preserved with all bones articulated with each other, however, some sutures between the bones are difficult to observe due to breakage or poor surface preservation (Figs. 2, 4, 5 and 6).The skull is deeper dorsoventrally than it is transversely wide as in other tyrannosaurids 2 .The lateral profile of the skull is deeper than alioramins Alioramus 27,28 , and Qianzhousaurus 10 , and is similar to other large-bodied tyrannosaurids like Tarbosaurus, Tyrannosaurus, and Gorgosaurus 2 .The snout and the skull are both relatively narrower than that in large-bodied tyrannosaurids such as Gorgosaurus 2 , Tarbosaurus 29 , and Tyrannosaurus 29,30 .The length of the skull is 47.5 cm from the anterior tip of the premaxilla to the posteriormost point of the occipital condyle, much smaller-but proportionally deeper-than that of Qianzhousaurus, which is 90 cm in length 10 .In tyrannosaurids, the skull becomes deeper and more robust with increasing maturity or size 2,22 .6.3-6.9% of the overall basal skull length (Table 1), which is similar to ∼4-10% of other tyrannosauroids, except extremely low 2.2% of Qianzhousaurus 10,31 .The anterior margin of the premaxilla is oriented almost vertically in lateral view, similar to Tarbosaurus 29 , and Tyrannosaurus 32 .This same margin is sloped posterodorsally in 'Nanotyrannus' 1 .The premaxilla forms the anterior and ventral edge of the external naris, which is anteroposteriorly long with its long axis sloping anteriorly.Two small, deep fossae are located on the lateral surface of the premaxilla just lateral to the anteroventral border of the external naris (Fig. 6).The fossae, which are present on  both premaxillae, are not seen in other tyrannosaurs.Four conical, elongated premaxillary teeth are considerably smaller than the maxillary teeth, as in Xiongguanlong, Timurlengia, and other Late Cretaceous tyrannosauroids 2,33 .All premaxillary teeth are visible in anterior view, as is characteristic of tyrannosaurids 10 .Two small foramina are preserved along the ventral margin of the right premaxilla parallel to the tooth row (Fig. 5).The premaxilla is well-separated from the maxilla in lateral view, likely due to the taphonomic distortion, so the location of the subnarial foramen is unclear.

Maxilla
Both maxillae are well-preserved.The ventral margin of the right maxillae is incomplete.The better preserved left maxilla has a distinct and ventrally oriented convex curvature near its midpoint, as seen in Timurlengia and Late Cretaceous large-bodied tyrannosauroids 2,32,33 , contra to the straight or slightly convex in early-branching tyrannosauroids, such as Guanlong 20 , Dilong 21 , Xiongguanlong 34 , and Suskityrannus 33 .The curvature of the ventral margin is an ontogenetic feature with this margin having a much weaker convexity in juveniles than in adults, as is seen in Tarbosaurus 24,29,35 .The curvature of the ventral margin is more strong than in juvenile Tarbosaurus 24 , but as strong as in large-bodied adult tyrannosauroids, such as Tarbosaurus 29 , and Tyrannosaurus 32 .The main www.nature.com/scientificreports/body of the maxilla is deep and short as in other large adult tyrannosaurids such as Tyrannosaurus and Tarbosaurus 19 .Contra with the long and low maxilla in Alioramus remotus 28 , Alioramus altai 19 , and Qianzhousaurus 10 .The promaxillary fenestra is obscured in lateral view, it rotates to face anteriorly as in other tyrannosaurines 2 .Unlike other tyrannosaurines, the anterior margin of the maxillary fenestra does not contact the anterior rim of the antorbital fossa 2 .The main body of the maxillae tapers in depth posteriorly below the antorbital fenestra.
Although the outer surface of the maxillae is full of cracks, it is clear the surface does not develop the dorsoventrally trending grooves and ridges present on other tyrannosaurids, as is the case in adult Daspletosaurus (CMN 8506), Tarbosaurus, Tyrannosaurus 19 , and Zhuchengtyrannus 36 specimens.The dorsoventrally trending grooves are present both in juvenile and adult Tarbosaurus 24,29 .Anteriorly, the maxillae articulate with the premaxilla, and the contact margin is oriented dorsoventrally resulting in a rounded anteroventral corner, as in most tyrannosaurids 19 .This condition differs from the strong posterodorsal orientation of the premaxillary articulation margin and sharp anteroventral point in Alioramus remotus 28 , Alioramus altai 19 , and Qianzhousaurus 10 .Posteriorly, the main body of the maxilla is articulated with the jugal, with the dorsal half of the maxilla overlaid by the jugal.Some alveolar neurovascular foramina are preserved immediately dorsal and parallel to the tooth row.The antorbital fossa is extensive and extends beyond the antorbital fenestra anteriorly and ventrally, similar to the condition found in Alioramus altai 19 , and Appalachiosaurus 37 .However, this condition differs from large adult tyrannosaurids, such as Tyrannosaurus and Tarbosaurus, where there is little lateral exposure of the fossa below the antorbital fenestra 19 .
The maxillary fenestra is large and sub-rectangular in shape, similar to the adult Tarbosaurus 29 , in contrast, the fenestra is oval in juvenile Tarbosaurus 24 .This fenestra is located relatively anteriorly between the anterior edge of the antorbital fossa and fenestra.In contrast, the fenestra is located more centrally along the maxilla of Alioramus  www.nature.com/scientificreports/altai, Appalachiosaurus, Albertosaurus, Bistahieversor, Gorgosaurus, and juvenile tyrannosaurines 19 .The margin of the maxillary fenestra is widely separated from that of the external antorbital fenestra, as in small-bodied tyrannosauroids such as Raptorex 38 , and juvenile specimens of tyrannosaurids, like Tarbosaurus 24 .Although the skull is smaller, the fenestra is relatively larger than in Appalachiosaurus 37 , juvenile and adult Gorgosaurus 39,40 , and 'Nanotyrannus' 1 .The maxillary tooth row is not completely preserved.The first tooth of the right maxilla is small and incisiform, similar to the premaxillary teeth (Fig. 5), but the first tooth of the left maxilla seems as large as the second one (Fig. 4).

Nasal
The left and right nasals are fused medially into a single vaulted element, a characteristic feature of tyrannosauroids 19,41 .Anteriorly each nasal divides into two processes: the subnarial process and the premaxillary process, as in other tyrannosaurs 19 .Anteriorly, the premaxillary processes are separated on the midline.The nasal is excluded from the antorbital fossa by the maxilla and lacrimal, in contrast, the nasal forms a part of this margin in adult Tarbosaurus 24,29 .In dorsal view, the subnarial processes are lateral to the premaxillary processes.This condition is also seen in Gorgosaurus, Albertosaurus, some specimens of Tyrannosaurus (BHI 3033), and Alioramus altai 19 .In contrast, the subnarial process is nearly beneath the premaxillary process in Daspletosaurus (CMN 8506) and other specimens of Tyrannosaurus (CM 79057) 19 .The nasals don't show any separation in the anterior end.However, they are separated at the anterior and posterior ends in juvenile and adult Tarbosaurus 24 .
The dorsal margin is straight in lateral view, in contrast, the anterior half of the dorsal margin is vaulting in lateral view in adult Tarbosaurus 29 .The nasal has pronounced vaulting observed in large tyrannosaurids 42 .The dorsal external surface of the nasal is rugose with prominent developed nasal protuberances, as is the case in most other tyrannosaurids 2,32 .The protuberances are homologous to the rugosities seen in all Late Cretaceous tyrannosauroids, Alioramus remotus 28 , Appalachiosaurus 37 , Yutyrannus 22,34 , except Xiongguanlong 34 .The rugose region is developed in all preserved nasals and extends around the middle of the antorbital fenestra.The posterior protuberances are connected to form two successive separated low medium ridges (Figs. 4, 5 and 6: nr).The development of the nasal protuberances is an ontogenetic feature and the nasal becomes more pronounced in larger specimens 2 .The dorsal surface of the nasal is smooth in Raptorex 38 , and the juvenile Tarbosaurus 24 .The pronounced protuberances on the fused nasal of Asiatyrannus are similar to other adult tyrannosaurids, such as Tarbosaurus 29 , Tyrannosaurus 32 , and Qianzhousaurus 10 .

Lacrimal
Both both lacrimals are preserved, but the caudal vertebrae obscure the left and the right is incomplete (Figs. 5,  6).The bone is L-shaped in lateral view, with a short posterior process that is similar to that of Tyrannosaurus 1 .The angle between the anterior and ventral rami is acute, similar to Daspletosaurus, Tarbosaurus, Tyrannosaurus, and Alioramus altai 19 .The cornual process is inflated without a discrete hornlet similar to the Tyrannosaurus-Tarbosaurus clade 2 .The cornual process is inflated similar to that of Tyrannosaurus 32 , and is more inflated than in Appalachiosaurus 37 , and the adult specimen of Tarbosaurus 19,29 .However, the surface of the cornual process is smoother than that of Tarbosaurus 29 .The inflation is not developed in Raptorex 38 , or less pronounced in the juvenile specimen of Tarbosaurus 24 .The cornual process has a triangular horn in Yutyrannus 22 , albertosaurines 2,40 , Daspletosaurus 2 , Alioramus altai 19 , and Qianzhousaurus 31 .

Jugal
The lineation between the postorbital and jugal is a straight, obliquely angled line.This contact divides the postorbital bar diagonally in an anteroventral to posterodorsal direction.However, the line is distinctly bent in Daspletosaurus, Albertosaurus 2 , and Tarbosaurus 29 .The ascending process terminates at the anterodorsal rim of the infratemporal fenestra but does not contact the squamosal.The ascending process meets the squamosal and excludes the postorbital from the infratemporal fenestra anteriorly in the Tyrannosaurus 32 .The lateral surface of the ascending process is smooth, as in Albertosaurus, Bistahieversor, Gorgosaurus, Teratophoneus, juvenile tyrannosaurines, and Alioramus altai 19 .By contrast, this lateral surface is excavated by a broad concavity in large subadult and adult tyrannosaurines (Daspletosaurus, Tarbosaurus, Tyrannosaurus) 19 .The entire postorbital contact zone is demarcated posteriorly by a pronounced ridge, as in Alioramus altai 19 .The ridge is especially prominent in the dorsal half of the jugal.
Posterior to the ascending process, the jugal forms the ventral floor of the infratemporal fenestra.The posterior ascending process of the jugal has an acute dorsal tip and is overlapped by the postorbital anteriorly, as in Tyrannosaurus 32 .The jugal horn appears to be developed, but the broken surface of the horn makes the condition uncertain.The jugal horn is only well developed in Alioramus altai 19 , and ridge-like in Qianzhousaurus 31 .Similar to Alioramus altai 19 , the lateral surface of the jugal's external surface develops anteroposterior raised lineations.Along the ventral margin of the anterior ramus and ventral to the orbit, there is a ridge-like jugal accessory horn in the ventrolateral portion of the anterior process (Fig. 5).The accessory horn is not present in other tyrannosauroids including Raptorex 38 , the juvenile and adult of Tarbosaurus 24,29 .Anteriorly, the ventral margin of the anterior ramus has a ventral curvature anterior to the accessory horn.However, the anterior ventral margin is straight or curves slightly dorsally in other tyrannosauroids 2,30 , including juvenile and adult Tarbosaurus 24,29 .

Frontal
Both frontals are well-preserved (Figs. 5, 6).The frontals are sutured medially as in other tyrannosaurids 2 .In dorsal view, the contact with the postorbital is V-shaped, with a concavity located on the frontal, as in Tyrannosaurus 32 .However, the border between the prefrontal and the prefrontal is unclear.The posterior portion of Vol:.( 1234567890 www.nature.com/scientificreports/ the frontals slopes ventrally into the supratemporal space from the anterior and medial supratemporal borders with a low sagittal crest extending posteriorly to parietals.The supratemporal fossae in the frontals are short, as in 'Nanotyrannus' , whereas they are elongated in Tyrannosaurus 1 .There is only a short contribution of frontal to the sagittal crest, like in 'Nanotyrannus' .In contrast, there is a long frontal contribution to the sagittal crest in Tyrannosaurus 1 .The frontal has a small, notch-like orbital rim, as in later-diverging taxa 33 .In contrast, the frontal makes a wide contribution to the orbital rim in Guanlong, Dilong, and Suskityrannus 33 .

Parietal
As in other tyrannosaurids, the parietals possess a fused sagittal crest 2,32 .The nuchal crest is broken and not present along the parietals.The frontoparietal suture is distinct and transversely directed.The parietal slopes broadly laterally within the supratemporal space, forming the medial and medioposterior walls of the fenestrae.

Postorbital
The right postorbital is well-preserved and exposed, while the lateral surface of the left postorbital is obscured by caudal vertebrae.Although the anterior portion of the process is broken, the cornual process is swollen and is present as a rugose convex boss that is located posterodorsally to the orbit, as in mature specimens of all other tyrannosaurids 31 .This process is absent in basal tyrannosauroids, but present as well-developed, rugose bulges in tyrannosaurids 19 .The descending process is tongue-shaped and gracile, as in Alioramus altai 19 and 'Nanotyrannus' 1 .The descending process is straight and gracile, without an anteriorly expanded suborbital process.The descending process is anteroposteriorly broad and constricts the orbit in large individuals of Yutyrannus 22 , Tarbosaurus 29 , Tyrannosaurus 1 , and Albertosaurus 43 .The anterior expansion is incipient in the juvenile specimen of Tarbosaurus 24 , however, it is more conspicuous in the juvenile specimen of Tarbosaurus than in Asiatyrannus 24 .Although the anterior expansion is incipient in Gorgosaurus, the descending process curved anteriorly in juvenile and adult individuals 39,40 , in contrast, the anterior edge of Asiatyrannus is almost straight.The descending process reaches the floor of the orbit ventrally, however, the process terminated higher in juvenile and adult Tarbosaurus 24,29 .The lateral surface of the process has anterodorsally trending fine lineations.In contrast, the lineations are horizontally directed and dorsally convex in Alioramus altai 19 , Albertosaurus 39 , Qianzhousaurus 31 , Tarbosaurus 24 , and 'Nanotyrannus' 1 .The postorbital bar composed of the jugal and postorbital is slender, straight, and banded-shaped, with almost straight and parallel anterior and posterior margins in lateral view.Generally, the postorbital bar is broader and the anterior margin is convex anteriorly due to the development of the anterior expansion of the postorbital.In other tyrannosauroids without anterior expansion development, the anterior margin is concave anteriorly, such as Raptorex 38 , Daspletosaurus 30 , 'Nanotyrannus' 1 , and juvenile Tarbosaurus 24 .

Squamosal
The squamosal is broadly convex and slopes posteroventrally in lateral view.The anterolateral process is forked, with dorsal and ventral projections.The lateral surface is overlapped by the posterior ramus of the postorbital.
In lateral view, the dorsal projection of the squamosal is exposed and is wider than the ventral projection.The descending process forms the derived squamosal-quadratojugal flange that bisects the infratemporal fenestra, which may be a tyrannosaurid synapomorphy 2,32,41 .The squamosal component contacts with the postorbital bar anteriorly and bisects the infratemporal fenestra completely, as in Tyrannosaurus 32 , but not in juvenile and adult Gorgosaurus 39,40 , juvenile and adult Tarbosaurus 24,29 , Alioramus altai 19 , and Qianzhousaurus 31 .The anterior tip of the flange is rugose and bulbous.The long axis of the ventral process projects roughly anteroposteriorly, as in other tyrannosaurids 19 .Posteroventrally, a portion of the squamosal is broken, however, the preserved portion of the posterior end directs posteroventrally.In the posterodorsal region of the lateral margin, there is a prominent thin ridge that extends along the dorsal edge.The crest directs dorsally and marks the highest point of the squamosal, as in Tyrannosaurus 32 .However, the ridge is located along the lateral surface of the squamosal rather than its dorsal edge as in Tarbosaurus 29 and Alioramus altai 19 .

Quadratojugal
The anterior and posterior margins of the quadratojugal's ascending processes are both concave, making the quadratojugal hourglass-shaped in lateral view.This condition is similar to that observed in other tyrannosaurids 19 .
The posterior process is short and directs ventroposteriorly.In lateral view, the ventral edge of the quadratojugal is strongly concave.

Quadrate
The quadrate forms the mandibular condyle, which is transversely expanded.The condyle is extended posterior to the posteriormost extent of the paroccipital processes, contra to other tyrannosaurids 2 .

The occipital elements
The occipital condyle is incomplete.The sutural separation of the exoccipitals and basioccipital is only visible ventrally, lateral to the basal tubera.The paroccipital processes are broad.The basal tubera is deep and bilaterally narrow as in Daspletosaurus sp.(TMP 94.143.1) 39, however, the basal tubera is broad in Tyrannosaurus 32 .The basal tubera is separated along its ventral margins by a concave notch that is deep and more than half of the depth of the tubera-similar to the derived tyrannosauroids 44 , Alioramus remotus 28  www.nature.com/scientificreports/

Mandible
The mandible is preserved in a closed position with the cranium, thus making the dorsal portion of the mandible obscured by the cranium.The mandible is robust and deep as in those of adult tyrannosaurids.The dentary is more robust and deeper dorsoventrally than those of alioramins, such as Alioramus 27 , and Qianzhousaurus 10 .Both dentaries are preserved with most of the lateral right dentary and ventral portion of the medial left dentary exposed.The anterior edge of both dentaries is more vertically directed than those of alioramins, such as Alioramus 27 , and Qianzhousaurus 10 .The ventral edge of the dentary has only slight concave curvature, such as Zhuchengtyrannus 36 .
The surangular is badly damaged along the dentary and posterior margins of the external mandibular foramen.Dorsal to the surangular foramen, the lateral surangular shelf is prominent and overhangs its lateral surface, as in Tyrannosaurus 32 .The shelf extended to the posterior end.In contrast, the edge of the surangular ends posteriorly above the posterodorsal corner of the surangular foramen in 'Bagaraatan' 45 , Tyrannosaurus 32 , and Alioramus altai 19 .
The angular is also badly damaged.The anterior dorsal edge, which borders the ventral of the external mandibular fenestra, appears complete and has a convex lateral surface 19 .The ventral edge of the angular forms the ventral edge of the mandible and is convex in lateral view.The ventral border of the angular is broadly convex, and there are no prominent rugosities as reported in Tyrannosaurus 19,32 .

Postcranial skeleton
Femur.The right hindlimb is well-preserved, with the femur, tibia, fibula, and partial pes preserved (Fig. 1).Only a partial tibia, fibula, and metatarsals from the left hindlimb were recovered (Figs. 2, 7).As in most theropods, including tyrannosauroids, the femur has a posteriorly oriented concave curve in lateral view (Fig. 2) 2,25 .The head of the femur projects mostly medially but also slightly proximally (Fig. 1).The proximal end of the right femoral head and the greater trochanter are partially missing due to the eroded nature of the proximal femur.Based on the preserved femur, the head is not distinctly elevated relative to the greater trochanter.Due to the incompleteness of the lesser trochanter, the relative height of the lesser and greater trochanters is indeterminate.They were separated by a deep, narrow cleft.The medullary cavity is expansive and cortical bone is relatively thin as in other tyrannosauroids 25 .In proximal view, the femoral head keeps a relatively constant width as it extends medially, similar to Gorgosaurus and Tarbosaurus 19 .A well-developed sulcus that would have housed ligaments is present along the midline of the posterior surface of the femoral head, as is observed in Alioramus altai 19 .The subtriangular fourth trochanter is powerfully developed along the posteromedial shaft, as in other tyrannosauroids 2,25 .
Tibia.Both tibiae are incomplete, with the right tibia being the most complete with only the distal articular condyles missing (Figs. 1, 2).The preserved length of the right tibia is shorter than the length of the right femur.Extensive hollowing is found throughout the tibia with the wall of the tibia being thin and the marrow cavity being relatively large.
Fibula.The fibular shaft is tightly appressed to the tibia as in Bistahieversor, and Moros 25 .The proximal half of the right fibula and the midshaft of the left fibula are preserved, with both articulating with the corresponding tibia (Figs. 1, 2).The iliofibularis tubercle is pronounced on the margin where the anterior and lateral surfaces meet, This iliofibularis tubercle, is large, prominent, rugose, and elongated proximodistally as in most tyrannosauroids like Dryptosaurus 46 .The fibular shaft is most constricted distal to the iliofiburlaris tubercle in lateral view.
Metatarsus and pes.The left and right metatarsals are preserved in articulation but are incomplete, with the mid-shaft section of the left metatarsal II-IV and the distal portion of right metatarsal III and IV being preserved (Figs. 1, 7).As in other tyrannosauroids, the metatarsus is long and slender 2 .The metatarsus is arctometatarsalian, with the metatarsal III pinched between metatarsals II and IV proximally, as in all tyrannosaurids 2,19 , and some non-tyrannosaurids such as Appalachiosaurus 37 , Bistahieversor, Dryptosaurus 46 , and Raptorex 38 .In contrast, a more normal theropod metatarsus, in which metatarsal III is large and un-pinched, is seen in the early branching tyrannosauroids, Kileskus 7 , Dilong 21 , Eotyrannus 47 , Guanlong 20 , and Yutyrannus 22 .The pinched proximal shaft of the metatarsal III is visible in anterior view.The whole preserved portion of metatarsal III is unobservable in posterior view, with metatarsal III and IV almost contacting each other.This condition where metatarsal III is proximally constricted is observed in tyrannosaurids and some non-tyrannosaurid tyrannosauroids.However, the proximal end of metatarsal III does not reach the proximal articulate surface with the tarsus, most likely due to poor preservation.The right metatarsal III is exposed in medial view, though its proximal portion missing.The preserved proximal portion is narrow in medial view, and it flares smoothly distally where it reaches its great width around the midpoint of the preserved shaft.The left surface shaft is inflated.The shaft has a constricted neck just proximal to the condyle.The medial ligament pit is large, deep, circular, and well-developed on the medial side of the condylar region.
Some nondescript pedal phalanges are preserved disarticulated and located beside the right metatarsus, with one exposed in dorsal view and two exposed in lateral view.They are considered to be from the right pes since they are non-ungual phalanges.In the dorsal exposed phalanx, it is constricted in the middle, with expanded proximal and distal articular regions.
Asiatyrannus is a tyrannosaurid theropod based on its possessing the following features: four incisiform premaxillary teeth, fused nasals, infratemporal fenestra constricted by the squamosal-quadratojugal flange, and the arctometatarsalian foot 2 .Both Asiatyrannus and Qianzhousaurus are discovered in the Nanxiong Formation of Nankang District, Ganzhou City 10 .Asiatyrannus differs from Qianzhousaurus with a proportionally deeper snout, longer premaxilla, deeper maxilla, and deeper dentary, and the cornual process of the lacrimal is inflated without a discrete horn 10 .Generally, the skulls of tyrannosauroids become deeper as their size increases, however, the Asiatyrannus skull (47.5 cm) is only around half in length as Qianzhousaurus (90 cm) but is proportionally deeper.Asiatyrannus is the first definitive deep-snout tyrannosaurid dinosaur from southern China and is the southernmost occurrence of tyrannosaurids in Asia.
The phylogenetic analysis places Asiatyrannus xui as a derived tyrannosaurid deeply nested in Tyrannosaurinae, positioned apart from the long-snouted alioramins and other early branching tyrannosaurines, and closer to the largest deep-snouted Tarbosaurus, Tyrannosaurus, and Zhuchengtyrannus (Fig. 8).In the latest Cretaceous, tyrannosaurids became restricted to Asia and North America, and they developed into large-bodied, bone-crunching apex predators-as exemplified by Tarbosaurus, Tyrannosaurus, and Zhuchengtyrannus 4,33,36 .Among tyrannosaurids, 'Nanotyrannus' (skull length 57.2 cm) and Nanuqsaurus (estimated skull length 60-70 cm in mature individuals), both larger than Asiatyrannus, might also represent the small to medium-bodied tyrannosaurids 48,49 .However, the taxonomic status of 'Nanotyrannus' is highly controversial, with the interpretation of 'Nanotyrannus' as a juvenile Tyrannosaurus being widely accepted 1, 30,48,50 .Thus Nanuqsaurus might be the small-bodied tyrannosaurid that inhabited in high-latitude of North America 49 .In seven of our eight MPTs, Asiatyrannus and Nanuqsaurus were recovered as sister taxa, however, only one character of the small body size (char 1) grouped them together.New material from the Prince Creek Formation suggests Nanuqsaurus has an Thus, Asiatyrannus currently represents the only definitive small to medium-bodied tyrannosaurine.More than one tyrannosaurids were discovered in several well-sampled assemblages, such as the Dinosaur Park Formation, the Judith River Formation, and the Nemegt Formation 1 .Asiatyrannus is part of the diversification of deep-snouted tyrannosaurids and emphasizes the higher species richness of theropods, especially tyrannosaurids, in southeastern China.The discovery reveals the coexistence of the long-snouted and deepsnouted tyrannosaurid in southeastern China as in the Late Cretaceous of central Asia 27 .However, the body size of the deep-snouted and long-snouted tyrannosaurids is the opposite in southeastern Asia compared to central Asia.Asiatyrannus is a small to medium-sized tyrannosaurine, with an estimated total body length of approximately 3.5-4 m.The histological study demonstrated that the individual is not a fully growth adult, however, Asiatyrannus is less than half the size of the other large-bodied tyrannosaurines in similar growth stages.Also, Asiatyrannus is only half the size of the coeval long-snouted Qianzhousaurus 10 .Asiatyrannus and Qianzhousaurus have different skull proportions and body sizes, suggesting they may occupy different ecological niches.In the Campano-Maastrichtian of eastern/central Asia and Laramidia, the large carnivore guilds are monopolized by tyrannosaurids, with adult medium-sized predators rare or absent 52 .Holtz 52 interpreted the "missing middlesized" niches in the theropod guilds of Late Cretaceous Laramidia and Asia may have been assimilated by juvenile and subadults of tyrannosaurid species.In southeastern China, the Qianzhousaurus undoubtedly occupied the apex predator 10 , but Asiatyrannus might represent the small to medium-sized theropod niche between the largebodied Qianzhousaurus and the diversified small-bodied oviraptorosaurs.

Histological sectioning
To assess the terminal age, growth rate, and ontogenetic stage of the holotype of Asiatyrannus xui (ZMNH M30360), we processed a histological thin section from the mid-diaphyseal of the right fibula.We prepared the histological thin section in the Key Laboratory of Vertebrate Evolution and Human Origin of Chinese Academy of Sciences, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences (IVPP).The section methodology followed the procedure described by Zhao et al. 53 .The samples were impregnated with resin and placed in a vacuum chamber to reduce the formation of air bubbles for approximately 12 h and were then embedded in Technovit 7200 VLC light curing 1-component-resin, and cut transversely at the mid-diaphysis of the limb bones.Sectioned surfaces were smoothed with grinding powder and afterward glued onto a glass slide with Araldite and ground to a thickness of 60 to 100 μm.Finally, a glass cover slip for sample protection.The sections were examined in normal light and cross-polarized light and imaged using the Zeiss Axio Imager 2 MAT Upright Microscope with Zen pro 3.7 software in IVPP.

Figure 1 .
Figure 1.Fossil remains of Asiatyrannus xui (ZMNH M30360).(A) Skeletal outlines showing recovered elements in yellow color.The skeletal reconstruction is the proportional scaling of Lythronax argestes drawn by Scott Hartman from Loewen et al. 23 ; (B) the skull in right lateral view; (C) the caudal vertebrae in left lateral view; (D) the right femur in posterior view; (E) the right tibia and fibula in posterior view; (F) The distal portion of the right metatarsal III in medial view and metatarsal IV in anterior view; (G) the middle shaft of the left tibia and fibula in anterior view; (H) The middle shaft of the left metatarsals in anterior view.ft the fourth trochanter; ift iliofibularis tubercle; mt metatarsal.

Figure 3 .
Figure 3.The thin section of the right fibula of Asiatyrannus xui (ZMNH M30360) in normal light (A) and cross-polarized light using a lambda compensator (B), inset boxes C'-F' indicate the position of the detailed pictures in (C-F); the detail of right fibula in normal light (C, E), and in cross-polarized using a lambda compensator (D, F).Black arrows denote lines of arrested growth (LAGs).

Table 1 .
Selected measurements of the holotype of Asiatyrannus xui (ZMNH M30360).The asterisk indicates a measurement from an incomplete element due to damage.